[FIX] sale: have online salesperson default working

A sales order on the website will have as salesperson:

- the contact salesperson, and if not set

But if it's not a new address, the "Online orders" > "Salesperson" is
not used.

With this changeset, the "Online orders" > "Salesperson" is used by
default if there is no salesperson on the partner as was the intention.

opw-2274665
closes #52933

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>